In the operation of a nuclear reactor, the fissionable nuclear fuel is typically contained in a fuel rod bundle or assembly. The fuel in the reactor is useful for a period of time and then becomes depleted and is no longer useful for a fission reaction in its present form. The nuclear fuel may, however, be reprocessed by techniques known in the art to yield fissionable material once again. Since reprocessing facilities are few and far between, spent nuclear fuel assemblies must often be transported for great distances.
Although the spent nuclear fuel is no longer useful in fission reactions, it is nevertheless still "hot" in terms of both radioactivity and temperature. In order to render the fuel assembly safe for handling outside the reactor vessel, the fuel assembly is usually loaded into a shipping cask. U.S. Pat. No. 3,962,587 (incorporated herein by reference) discloses such a shipping cask.
The shipping casks generally comprise a body for containing the spent nuclear fuel assembly therein. The body of the cask is typically made of a heavy shielding material to absorb radiation from the material inside. The shipping cask is usually loaded by lowering into a storage pool at the nuclear power plant. When the cask is removed from the pool, it contains both the fuel assembly and water from the pool, which may or may not be drained off later. The loaded cask is extremely heavy and since it contains highly radioactive material, measures must be taken to insure that the cask is not dropped. In order to provide the desired extra measure of safety required, the trunnions attached to the cask for lifting the cask are usually oversized to withstand several times the normal load levels. Inspection is usually required against material failure and/or attachment failure, such as a defect in a weld. Thorough inspection is often difficult or impossible, and is always a costly expedient.
Another expedient is to provide a redundant number of trunnions, along with a hoisting yoke which attaches to all trunnions and which transfers the load to the remaining trunnions if one trunnion should fail. That expedient is also expensive, and the added presence of the yoke impedes the loading and unloading of spent fuel assemblies.
The shipping cask is typically transported by railroad car or by truck. Since the possibility of a crash is always present, precautions must be taken to minimize the probability of a cask rupture upon impact with an obstacle. Impact limiters have been used in the past to minimize this probability. An impact limiter generally comprises a cap-like structure filled with a crushable material which fits on the ends of the cask. Upon impact the crushable material absorbs a large portion of the impact energy, thereby reducing the forces applied to the body of the cask. Controlled absorption of the impact is a desirable feature of such impact limiters.